This invention relates to a capacitive ultrasound transducer, and more particularly to such a transducer with a porous layer and additionally to such a transducer with a doped conductor region electrode.
The best ultrasonic transducers for use in air are capacitive transducers, due to the low mass of the diaphragm actuator and efficient coupling of energy between acoustic and electrical forms. Former capacitive ultrasonic transducers are inefficient because they use a grooved backplate covered by a continuous metallized polymer diaphragm. This structure has high stray capacitance, and suffers from a large transducer gap in the center of the diaphragm where motion is the greatest, necessitating high voltages for actuation. Former transducers were highly resonant (narrow bandwidth) and cannot reproduce spread-spectrum signals such as short pulse signals, chirp signals or pseudo-random noise.
In one prior art approach an attempt was made to increase sensitivity by providing a separate reservoir connected to the gap by capillaries which permitted oil to move between the gap and reservoir in response to motion of the diaphragm. However, the relatively large reservoir and capillaries did not lend themselves to good high frequency performance. In addition the number and size of capillaries required for good sensitivity at high frequencies was practically unobtainable.
It is therefore an object of this invention to provide an improved capacitive ultrasound transducer.
It is a further object of this invention to provide such an improved capacitive ultrasound transducer which is more efficient.
It is a further object of this invention to provide such an improved capacitive ultrasound transducer which has more sensitivity as a receiver and greater output as a transmitter.
It is a further object of this invention to provide such an improved capacitive ultrasound transducer which operates on lower voltage.
It is a further object of this invention to provide such an improved capacitive ultrasound transducer which has broader bandwidth and can transmit and receive short pulse signals, chirp signals, and pseudo-random noise.
It is a further object of this invention to provide such an improved capacitive ultrasound transducer which employs an electrode formed as a conductive region in the diaphragm.
The invention results from the realization that an improved, more efficient, broader bandwidth capacitive ultrasound transducer which operates on lower voltage can be achieved by using a porous layer so that the gap between the porous layer and diaphragm is small and necessitates only a small operating voltage but the total fluid in the gap and in the porous layer is of much larger volume, as if the gap were much larger and provides a greater compliance resulting in a higher sensitivity at lower voltage, and the increased resistance to fluid movement afforded by the porous reservoir broadens the bandwidth of the transducer. Further benefits are realized from employing an electrode formed by a conductive region on the diaphragm.
This invention features a capacitive ultrasound transducer comprising a dielectric diaphragm with an electrode and a porous layer. A spacer structure between the diaphragm and porous layer defines a capacitive gap between them. The pores of the porous layer provide a compliant reservoir for the fluid in the gap.
In a preferred embodiment, the porous layer may include continuously connected porosity and it may be disposed on a support substrate. The support substrate may be silicon. The porous layer may include porous silicon or metal, for example from the group including aluminum, tin, nickel, titanium, stainless steel, brass, bronze, copper and zinc. The diaphragm may include silicon nitride, silicon oxide, Mylar, Kapton, or polysilicon. The spacer structure may be formed integrally with the porous layer. The electrode may include a metallized contact on the diaphragm or may include a doped conductive region on the diaphragm. The diaphragm may be made of polysilicon and the doped region may include silicon and a dopant from the group of boron, phosphorous, arsenic, antimony and aluminum. The porous layer may have a pore volume fraction, between 20% and 80%. The size of the pores may be not greater than the width of the capacitive gap. The capacitive gap may be between 0.1 and 200 microns. The spacer structure may be a dielectric and may be made integral with the porous layer or may be a discrete structure. The porous reservoir may be capable of absorbing substantially all of the volume of fluid in the gap. The gap may contain air or dielectric oil. The porous layer may be conductive.
The invention also features a capacitive ultrasound transducer including a semi-insulating diaphragm including a doped conductive region forming an electrode and a second layer. There is a spacer structure between the diaphragm and the second layer for defining a capacitive gap between them. The semi-insulating diaphragm may include one of a group including polysilicon and silicon carbide. The doped conductor region may include a dopant from the group of boron, phosphorous, arsenic, antimony and aluminum.